The separation of substances by membranes is advantageous from the standpoint of energy efficiency as compared with other separation methods and is characterized by the use of a small-sized light-weight apparatus, a simple mechanism and a maintenance-free operation. Thus, it is widely applied to various industries.
The separation membrane has the following basic requirements:
(1) Separation ability of separating the target substance from other components; PA1 (2) Permeability to substances; and PA1 (3) Properties such as strength, heat resistance, durability and solvent resistance PA1 (1) Separation membranes formed from polyhydrazidoimide resins; PA1 (2) The separation membrane according to Clause (1), having a non-porous dense layer formed from polyhydrazidoimide resins; PA1 (3) The separation membranes according to Clause (2), which are gas-separation membranes; PA1 (4) The separation membranes according to Clause (2), having a composite structure comprising a non-porous dense layer formed from polyhydrazidoimide resins and a porous supporting layer formed from materials different from the resin constituting said non-porous dense layer; PA1 (5) The separation membranes according to any one of Clauses (1) to (4), wherein said polyhydrazidoimide resins are polymers obtained from the reaction of aromatic tetracarboxylic dianhydrides and aromatic dihydrazides as essential components; PA1 (6) The separation membranes according to Clause (5), wherein said aromatic tetracarboxylic dianhydride is 4,4'-(hexafluoroisopropylidene)diphthalic anhydride; and PA1 (7) The separation membranes according to Clause (5), wherein said aromatic dihydrazide is isophthalic dihydrazide or terephthalic dihydrazide.
The separation properties of the membranes, if they are dense membranes, are essentially inherent to the material of the membranes and are primarily characteristics governing the yield of substances to be separated, i.e., characteristics governing the running cost.
On the other hand, the permeability properties of the membranes to substances are characteristics mainly governing the required area of membrane and the size of membrane module and apparatus, i.e., initial cost. Thus, industrially feasible properties can be realized by the development of materials having a high permeability to substances and the formation of the thin film of the dense permselective layer.
In general, the ability of the membrane to separate substances and the permeability of the membrane to substances conflict with each other. The development of membrane materials having an excellent balance between the two characteristics has been extensively made. Aromatic polymer materials have excellent gas permeabilities and excellent gas separation abilities. Extensive studies have been made using various aromatic polymer materials for gas separation particularly. Many studies focusing polyimides, polyimidazopyrrolones, etc. are reported in articles and reviews, e.g., Kobunshi Kako (Polymer Applications), vol. 41, page 16 (1992), Kobunshi (High Polymers, Japan), vol. 42, page 682 (1993), Polymer, vol. 35, page 4970 (1994), Journal of Membrane Science, vol. 88, page 37 (1994), Polymer Preprints Japan, vol. 43, page 2273 (1994), Hyoumen (Surface), vol. 33, page 308 (1995), Journal of Membrane Science, vol. 111, page 169 (1996).
Recently, it has been found that among a large number of polymer materials polyimides resin exhibited a good balance between gas separation properties and gas permeability and is excellent in properties such as strength, durability, heat resistance and solvent resistance. Extensive studies have been made of gas separation membrane and pervaporation membrane having a non-porous dense layer formed from polyimides.
On the other hand, a polyhydrazidoimide resin synthesized from pyromellitic dianhydride and isophthalic dihydrazide via precursor polyhydrazide is reported as a heat-resistant aromatic polyimide-amide in Journal of Polymer Science, Part B, Polymer Letters, vol. 3, page 679 (1965). Further, several kinds of polyhydrazidoimide resins as polyimide-amides are reported in Yuki Gosei Kagaku (Journal of Synthetic Organic Chemistry, Japan), vol. 23, page 1028 (1965), Die Makromolekulare Chemie", vol. 94, page 114 (1966), Bulletin of the Chemical Society of Japan", vol. 39, page 1795 (1966), and Kogvo Kagaku Zasshi (Journal of the Chemical Society of Japan, Industrial Chemistry Section), vol. 70, page 192 (1967). However, no reports have been made of a separation membrane formed from polyhydrazidoimide resins and their substance separation properties.
The permeability of gas separation membranes can be improved by reducing the thickness of the non-porous dense layer. However, the separation properties of gas separation membranes are attributed to the characteristics inherent to the materials constituting the membranes. Accordingly, if materials having excellent separation properties are not available, the gas separation membranes cannot fully compete with rival techniques such as low-temperature processing and Pressure Swing Adsorption (PSA).
Gas separation membranes which have already been put into practical use are generally used for low purity gases. Accordingly, it is said that in order to promote the use of a separation membrane in the application including high purity gas covered by rival techniques such as low-temperature processing and Pressure Swing Adsorption, materials which exhibit an oxygen/nitrogen separation factor of not less than 7 are needed. However, no separation membranes having separation properties which can be practically satisfied have ever been found.